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This series of images are discussed in our most recent Sciencepaper, Molecular Rulers for Scaling Down Nanostructures, A. Hatzor, P. S. Weiss, Science 291, 1019 (2001).
A series of field emission scanning electron microscope (FESEM) images showing gold dots of different sizes and shapes and a gold ring formed in the center of hollow parent structures.
A field emission scanning electron microscopy image showing a ~30 nm gold dot formed in the center of a hollow gold parent structure supported on an oxidized Si substrate. This nanostructure was fabricated by a 'molecular ruler' resist process developed to extend the range of conventional nanolithography techniques.
The scaling-down process forms a gold ring connected to two thin gold channels on opposite sides. The gold channel size is ~15 nm. The ring and channels are formed in between a parent circle and two L-shaped structures.
"Parent" gold hollow square of a different hole size. The square in the center was formed similarly to the circle shown in the first image.
Field emission scanning electron microscopy images depicting stages of a nanostructure reduction process. From top to bottom, gaps between "parent" gold traces on oxidized Si are reduced from ~110 nm to ~65 nm (third row left) and ~25 nm (third row right) by 10-layer and 20-layer molecular ruler resists, respectively. Thin metal wires ~65 nm (bottom left) and ~25 nm (bottom right) wide, respectively, are formed, separated by precisely determined gaps from each of the parent gold traces.
The interactions in this system are strong enough to slow down the rate ofdiffusion on the gold surface. This movie shows the motion of single-atomsteps over several scans:
When a mixture of different alkanethiols is deposited onto the gold substrate,sometimes the interactions between similar molecules allow the molecules tophase-segregate into domain structures. In these images, mixtures ofester-terminated and methyl-terminated alkanethiols were used.
Domains in a 1:3 mixed composition alkanethiol SAM
Domains in a 1:1 mixed composition alkanethiol SAM
Domain coalescence over a 37 minute interval in a 3:1 mixed composition SAM
However, the following images show random distribution in other mixedmonolayers. The first example shows a mixture of two methyl-terminated alkanethiols of different lengths, while the second example shows a mixture of an alkanethiol chain with an arenethiol chain. The failure to coalesce in these cases may be due to weaker interactions between the chains, orbecause of the greater concentration ratios in the mixtures.
3-D view of the previous image
Random distribution in a mixed composition alkanethiol/arenethiol SAM
2.4M MPEG of wire-like molecules inserted into an alkanethiol self-assembled monolayer
Other methods of monitoring surface motion with the STM include cooling thesample to slow the diffusion rate, and measuring the residence time ofspecies at specific surface sites. Both of these techniques have been used in our studies of benzene molecules on Cu{111}.
3-D close up of the previous image
We have also developed a microscope capable of studying insulating substrates,by using an alternating current to tunnel electrons back and forth betweenthe surface and the tip. This ACSTM produced these images of a lead silicatemicrochannel plate. The hole in the center of the image is a defect in thesurface; the edge of one of the channels is shown as a giant cliff.
Inside the defect in the previous image
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